Method and apparatus for image forming and computer program product

ABSTRACT

An image forming apparatus capable of minimizing degradation in density correction as a part of skew correction includes a noise-occurrence determining unit, a correction-target-pixel selecting unit, a density correcting unit, and a phase correcting unit. When the noise-occurrence determining unit determines that noise would appear, the density correcting unit performs density correction on a correction target pixel that is determined by the correction-target-pixel selecting unit. The phase correcting unit corrects an output point in time of image data of the density-corrected correction target pixel in a pixel period during which the image data can be output so as to output the image data at a position displaced away from a shift position or toward a shift position.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by referencethe entire contents of Japanese priority document 2008-068202 filed inJapan on Mar. 17, 2008 and Japanese priority document 2009-016733 filedin Japan on Jan. 28, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technology for correcting colormisalignment in an image forming apparatus.

2. Description of the Related Art

Occurrence of color misalignment is a critical problem in color imageforming apparatuses. For this purpose, typical color image formingapparatuses have a function of detecting and reducing colormisalignment. To implement such a function, in one approach, tonerpatterns of different colors are formed on a transfer belt, those tonerpatterns are detected with a photosensor, amounts of color misalignmentfor various causes are calculated based on the result of detection ofthe tonner patterns, and a feedback control is performed based on thecalculated amounts of color misalignment. Examples of the causes includemain-scanning-direction misregistration, sub-scanning-directionmisregistration, main-scanning-direction magnification error, and skew.A feedback correction for compensating the calculated misalignmentamounts is then performed to reduce the misalignment.

A color image forming apparatus performs the feedback correction atvarious occasions to constantly limit each misalignment amount below apredetermined value. Such feedback correction is performed, for example,when the image forming apparatus is turned on, when the image formingapparatus undergoes an environmental change such as a temperaturechange, and when a print count of the image forming apparatus reaches apredetermined number.

Examples of the method for correcting the color-to-color misalignmentwill be explained below. The main-scanning-direction misregistration andthe sub-scanning-direction misregistration can be corrected by adjustingwrite-start timing of a laser beam on a photosensitive drum.

The main-scanning-direction magnification error can be electricallycorrected by adjusting a pixel clock.

Skew of a laser beam that performs scanning exposure can be correctedmechanically, or by using an image processing technique. The method ofmechanically correcting the skew uses an adjusting mechanism that isused to adjust a position of a mirror inside a laser-beam write unit tocorrect the skew. However, to implement this method automatically, anactuator such as a mirror-displacing motor is required to move themirror, which means additional cost. This method is furtherdisadvantageous in making it difficult to configure the laser-beam writeunit compact.

The image processing technique for correcting the skew of a laser beamis as follows. A portion of image data is stored in a line memory thathas a capacity to store therein one line of image data in themain-scanning direction. Pixels belonging to the one line of the imagedata in the line memory are then divided into a plurality of pixelblocks. When reading (outputting) the image data in each of the pixelblocks in the line memory, the order of reading the image data in eachof the pixel blocks is changed so that the image data is shifted in adirection opposite to a skew direction. Accordingly, color-to-color skewcan be corrected. Because this method requires only one additional linememory of a size corresponding to a desired correction area, this methodis advantageous in being implementable with a relatively smalladditional cost as compared with that of the mechanical correctingmethod. This correcting method based on the image processing techniqueis effective not only for skew correction but also for reducing thedegree of distortion resulting from the property of a lens in thelaser-beam write unit or the like.

However, the method based on the image processing technique isdisadvantageous in that because relation between neighboring pixels on ashift position changes, a color density can be locally increased ordecreased. This can result in banding noise extending in thesub-scanning direction on an output image (for example, an image printedon a printing paper). In particular, such local color density increaseor decrease frequently occurs on an image that is processed by using adigital halftoning method such as dithering, and produces banding noiseextending in the sub-scanning direction.

Japanese Patent No. 3715349 discloses a conventional technique forcorrecting skew of an image and reducing banding noise that can resultfrom the skew correction. In the conventional technique, it isdetermined whether a pixel of interest is at a shift position. If thepixel of interest is at the shift position, and when a neighboring pixelin the main-scanning direction of the pixel of interest has changed anda pixel pattern of pixels in the vicinity of the pixel of interestmatches with a predetermined pattern, density correction is performedfor the pixel of interest. This density correction is performedaccording to a set of the matched pattern and the position where theneighboring pixel changes.

Although the conventional technology disclosed in Japanese Patent No.3715349 teaches to perform the density correction of the pixel ofinterest, it does not teach output timing of the pixel of interest in apixel period during which the pixel of interest having undergone thedensity correction can be output. Accordingly, the conventionaltechnology is disadvantageous in that the density correction performedon the pixel of interest can lead to color density change, althoughwhich is a small change, even in a portion where banding noise resultingfrom the density correction does not occur.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

According to an aspect of the present invention, there is provided animage forming apparatus including a skew correction unit that performsskew correction on image data by dividing pixels that belong to one linein a main-scanning direction of the image data into pixel blocks at atleast one shift position and shifting a pixel block of the pixel blocksin a sub-scanning direction against a direction of skew; a noisedetermining unit that determines whether a pixel of interest that is onthe shift position is a noise-inducing pixel, the pixel of interestbeing each pixel on the shift position, the noise-inducing pixel being apixel that leads to a local color density change because of a change inrelationship with an adjacent pixel of the noise-inducing pixel; aselecting unit that selects, when the pixel of interest is determined asbeing the noise-inducing pixel, any one of the pixel of interest and apixel in vicinity of the pixel of interest as a correction target pixelon which color density correction is to be performed; a first correctingunit that performs the color density correction ondensity-not-yet-corrected image data of the correction target pixel toacquire density-corrected image data; and a second correcting unit thatcorrects an output point in time at which the density-corrected imagedata is to be output, the output point in time falling within a pixelperiod during which the density-corrected image data of the correctiontarget pixel can be output, such that the image data of the correctiontarget pixel is output at an output position that is displaced from acenter position corresponding to a center point in time of the pixelperiod in any one of a direction toward the shift position and adirection away from the shift position.

According to another aspect of the present invention, there is providedan image forming method including performing skew correction on imagedata by dividing pixels that belong to one line in a main-scanningdirection of the image data into pixel blocks at least one shiftposition and shifting a pixel block of the pixel blocks in asub-scanning direction against a direction of skew; determining whethera pixel of interest that is on the shift position is a noise-inducingpixel, the pixel of interest being each pixel on the shift position, thenoise-inducing pixel being a pixel that leads to a local color densitychange because of a change in relationship with an adjacent pixel of thenoise-inducing pixel; selecting, when the pixel of interest isdetermined as being the noise-inducing pixel at the determining, any oneof the pixel of interest and a pixel in vicinity of the pixel ofinterest as a correction target pixel on which color density correctionis to be performed; performing the color density correction ondensity-not-yet-corrected image data of the correction target pixel toacquire density-corrected image data; and correcting an output point intime at which the density-corrected image data is to be output, theoutput point in time falling within a pixel period during which thedensity-corrected image data of the correction target pixel can beoutput, such that the image data of the correction target pixel isoutput at an output position that is displaced from a center positioncorresponding to a center point in time of the pixel period in any oneof a direction toward the shift position and a direction away from theshift position.

According to still another aspect of the present invention, there isprovided a computer program product that includes a computer-readablerecording medium and a computer program stored on the readable recordingmedium, the computer program when executed on a computer causes thecomputer to execute the above image forming method.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view of an image forming unit, an exposureunit, and a transfer belt for explaining an image forming principle of acolor copier according to a first embodiment of the present invention;

FIG. 2 is a perspective view of a transfer belt, on which correctionpatterns are formed, of the color copier depicted in FIG. 1;

FIG. 3 is a block diagram of an exemplary configuration of a mechanismthat performs write control and misalignment correction in the colorcopier depicted in FIG. 1;

FIG. 4 is a block diagram of an exemplary configuration of a writecontrol unit in the mechanism depicted in FIG. 3;

FIG. 5 is a flowchart for explaining correction of color misalignment;

FIG. 6 is a flowchart of an exemplary method for printing implemented bythe color copier depicted in FIG. 1;

FIG. 7 is a timing chart for explaining write timing correction in thesub-scanning direction performed by the write control unit depicted inFIG. 4;

FIG. 8 is a schematic diagram of exemplary correction patterns formed onthe transfer belt depicted in FIG. 2;

FIG. 9 is a schematic diagram for explaining a method of calculating askew amount;

FIG. 10 is a table of exemplary skew amounts relative to a referencecolor K (black) for a resolution of 600 dots per inch (dpi) in thesub-scanning direction;

FIG. 11 is a table of exemplary skew correction amounts obtained fromthe skew amounts in the table of FIG. 10;

FIGS. 12 to 17 are schematic diagrams for explaining a first skewcorrection method by way of an example;

FIGS. 18 to 23 are schematic diagrams for explaining a second skewcorrection method by way of an example;

FIGS. 24 and 25 depict a timing chart of read/write timings for readingand writing in the sub-scanning direction performed by the write controlunit depicted in FIG. 4 during skew correction;

FIG. 26 is a schematic diagram of a pixel of which toner area coverageis changed by corrective shifting;

FIG. 27 is a block diagram of a skew-correction processing unitaccording to the first embodiment;

FIG. 28 is a block diagram of a noise-correction processing unit of theskew-correction processing unit depicted in FIG. 27;

FIG. 29 is a flowchart for explaining how the noise-correctionprocessing unit depicted in FIG. 29 performs density correction andphase correction of image data;

FIG. 30 is a schematic diagram for explaining how a pixel of interestbecomes a noise-inducing pixel by way of an example;

FIG. 31 is a schematic diagram of an example of pixels in the vicinityof a shift position;

FIG. 32 is a schematic diagram of an example of pixels in the vicinityof the shift position;

FIG. 33 is a schematic explanatory diagram of pixels in the vicinity ofa shift position before and after corrective shifting;

FIG. 34 is another schematic explanatory diagram of pixels in thevicinity of a shift position before and after corrective shifting;

FIG. 35 is a schematic explanatory diagram of a correction target pixelthat is determined based on pixels in the vicinity of a shift position;

FIG. 36 is a schematic explanatory diagram of a correction target pixelthat is determined based on pixels in the vicinity of a shift position;

FIG. 37 is a schematic explanatory diagram of neighboring pixels forwhich color density determination is performed;

FIG. 38 is a schematic explanatory diagram of upper, lower, left, andright neighboring pixels of the correction target pixel depicted in FIG.35;

FIG. 39 is a schematic explanatory diagram of pixel sizes of the upper,lower, left, and right neighboring pixels depicted in FIG. 38;

FIGS. 40 to 43 are schematic diagrams for explaining how color densitycorrection of image data of a correction target pixel is performed byway of examples;

FIG. 44 is a table in which color densities of image data of neighboringpixels of a correction target pixel and density correction valuestherefor are mapped together;

FIG. 45 is a schematic diagram for explaining how a color density ofimage data of a correction target pixel is corrected by way of anexample;

FIG. 46 is a schematic diagram of an exemplary pixel arrangement in astate before it is subjected to corrective shifting;

FIG. 47 is a schematic diagram of the pixel arrangement depicted in FIG.46 of which right side has been shifted downward;

FIG. 48 is a schematic diagram for explaining ideal density correctionfor compensating an increase in color density;

FIGS. 49 and 50 are schematic diagrams for explaining conventionaldensity correction for compensating an increase in color density;

FIG. 51 is a schematic diagram of an exemplary pixel arrangement in astate before it is subjected to corrective shifting;

FIG. 52 is a schematic diagram of the pixel arrangement depicted in FIG.51 of which right side has been shifted upward;

FIG. 53 is a schematic diagram for explaining ideal density correctionfor compensating a decrease in color density;

FIGS. 54 and 55 are schematic diagrams for explaining conventionaldensity correction for compensating a decrease in color density;

FIGS. 56 and 57 are schematic diagrams for explaining output-time pointcorrection according to the first embodiment for compensating anincrease in color density; and

FIGS. 58 and 59 are schematic exemplary diagrams for explainingoutput-time point correction according to the first embodiment forcompensating a decrease in color density.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are described in detailbelow with reference to the accompanying drawings. Color copiers will bedescribed below as specific examples of image forming apparatusesaccording to the embodiments. However, applications of the presentinvention are not limited to color copiers, and the present inventioncan be applied to any apparatus that performs skew correction by meansof image processing. Examples of such an apparatus include a facsimileand a multifunction product (MFP) that performs more than one functionin a single casing, such as copying, faxing, scanning, and printing. Theconfiguration of a color copier and skew correction according to anembodiment of the present invention will be described first, which willbe followed by descriptions about the configuration and the skewcorrection that feature the present embodiment.

A principle of image forming to be performed by a color copier 60according to a first embodiment of the present invention will bedescribed with reference to FIG. 1. As depicted in FIG. 1, the colorcopier 60 includes an image forming unit 1, an exposure unit 9, and atransfer belt 3. The color copier 60 forms images on a transfer sheet byusing the technique of electrophotography.

The color copier 60 is a what is called tandem-type image formingapparatus, moreover, the color copier 60 employs a direct transfermethod. In this color copier 60, image forming units 1Y, 1M, 1C, and 1Kthat form images of four colors (yellow (Y), magenta (M), cyan (C), andblack (K)) are linearly arranged in the direction of movement of theendless transfer belt 3. The transfer belt 3 conveys a transfer sheet 2sequentially from under the image forming unit 1Y to the image formingunit 1K. The image forming units 1Y, 1M, 1C, and 1K are parts of theimage forming unit 1. Meanwhile, in the following description, unitsbeing identical in configuration but different from one another only intoner color will be referred to with a reference symbol indicating thecolor omitted from its reference numeral in some cases. For example, theterm “the image forming unit 1” will be used below to denote anarbitrary one of the image forming units 1Y, 1M, 1C, and 1K. Thetransfer sheet 2 is a transfer medium such as a printing paper.

The transfer belt 3 is wound around a drive roller 4 that rotates as adriving member and a driven roller 5 that is rotated by the drive roller4. As the drive roller 4 rotates, the transfer belt 3 is rotated. One ormore pieces of the transfer sheets 2 are stacked in a paper feed tray 6that is arranged below the transfer belt 3. The topmost one of thetransfer sheets 2 is fed toward the transfer belt 3 by using a not shownconveying mechanism. The transfer sheet 2 sticks onto the surface of thetransfer belt 3 by electrostatic attraction. The transfer sheet 2 isthen conveyed to a position that is under the image forming unit 1Y toform a Y-toner image on the transfer sheet 2.

The image forming unit 1 includes a photosensitive drum 7 (7Y, 7M, 7C,7K), an electrostatic charger 8 (8Y, 8M, 8C, 8K), a developing unit 10(10Y, 10M, 10C, 10K), a photosensitive drum cleaner 11 (11Y, 11M, 11C,11K), and a transfer unit 12 (12Y, 12M, 12C, 12K). The electrostaticcharger 8, the developing unit 10, the photosensitive drum cleaner 11,and the transfer unit 12 are arranged around the photosensitive drum 7.

The surface of the photosensitive drum 7Y of the image forming unit 1Yis uniformly charged by the electrostatic charger 8Y, thereafter exposedto a laser beam LY in the exposure unit 9 for a yellow toner image.Hence, a latent image is formed on the surface of the photosensitivedrum 7Y. The developing unit 10Y develops this latent image and forms atoner image on the photosensitive drum 7Y. The transfer unit 12Ytransfers this toner image onto the transfer sheet 2 at a position(transfer position) where the photosensitive drum 7Y comes into contactwith the transfer sheet 2 on the transfer belt 3. Hence, a single-colori.e., yellow, image is formed on the transfer sheet 2. Residual toner isremoved from the photosensitive drum 7Y, from which the image has beentransferred, by the photosensitive drum cleaner 11Y to prepare thephotosensitive drum 7Y for subsequent image forming.

The transfer sheet 2 onto which the single-color (Y)-toner image hasbeen transferred by the image forming unit 1Y is conveyed to the imageforming unit 1M by the transfer belt 3. In the image forming unit 1M, anM-toner image is similarly formed on the photosensitive drum 7M andtransferred in a superimposed manner onto the transfer sheet 2. Thetransfer sheet 2 is subsequently conveyed to the image forming units 1Cand 1K in this order where a C-toner image and a K-toner image aresimilarly formed, respectively, and transferred onto the transfer sheet2 to form a full-color image on the transfer sheet 2.

When the transfer sheet 2 on which the full-color image is formed comesout of the image forming unit 1K, the transfer sheet 2 is peeled awayfrom the transfer belt 3. The full-color image is fixed onto thetransfer sheet 2 in a fixing unit 13. Thereafter, the transfer sheet 2is discharged out of the color copier 60.

Occurrence of color misalignment is a critical problem in tandem-typecolor image forming apparatuses. How the color copier 60 correctscolor-to-color misalignment will be described below.

When performing color-to-color misalignment, correction patterns 14 ofthe four colors of are formed on the transfer belt 3 in the mannerexplained above. Optical detection sensors 15 and 16 detect thecorrection patterns 14 and output detection signals for use incalculation of color-to-color misalignment amounts for each of thevarious causes of the color-to-color misalignment. The causes of thecolor-to-color misalignment can be main-scanning-directionmisregistration and sub-scanning-direction misregistration,main-scanning-direction magnification error, and skew. The main-scanningdirection is orthogonal to the direction of movement of the transferbelt 3, i.e., parallel to rotation axes of the photosensitive drums 7Y,7M, 7C, and 7K. The sub-scanning direction is parallel to the directionof movement of the transfer belt 3, i.e., perpendicular to the rotationaxes of the photosensitive drums 7Y, 7M, 7C, and 7K. The misalignmentamounts are compensated for each of the causes to correct themisalignment. The color copier 60 corrects color-to-color misalignmentby using the correction patterns 14 prior to actually forming afull-color image on the transfer sheet 2.

FIG. 2 is a perspective view of the transfer belt 3 on which thecorrection patterns 14 are formed. In the color copier 60, the imageforming units 1Y, 1M, 1C, and 1K form the correction patterns 14 for usein correction of color-to-color misalignment on the transfer belt 3. Thecorrection patterns 14 are detected by the detection sensors 15 and 16.In the example depicted in FIG. 2, the detection sensors 15 and 16 arearranged on opposite ends of the transfer belt 3 in the main-scanningdirection. The correction patterns 14 are formed on the transfer belt 3at positions corresponding to the detection sensors 15 and 16. While thecorrection patterns 14 are moved by rotation of the transfer belt 3 inthe direction depicted in FIG. 2, the correction patterns 14 aredetected by the detection sensors 15 and 16 when the correction patterns14 pass through detection areas of the detection sensors 15 and 16. Whenthe correction patterns 14 are detected, various misalignment amountsare obtained by calculations based on the results of the detection.Examples of the misalignment amounts include main-scanning-directionmagnification error amounts, main-scanning-direction misregistrationamounts, sub-scanning direction misregistration amounts, skew amounts,and distortion amounts. Misalignment correction amounts for each ofthese misalignment components are calculated from the misalignmentamounts.

A block diagram and operations relevant to control of the color copier60 will be described below. FIG. 3 is a block diagram of an exemplaryconfiguration of a mechanism of the color copier 60 that performs writecontrol and misalignment correction. The color copier 60 that performthe misalignment correction includes, as processing units, the detectionsensors 15 and 16, a printer controller 111, a scanner controller 112,an engine control unit 113, and laser diode (LD) control units 114(114K, 114M, 114C, and 114Y).

The detection sensors 15 and 16 detect positions of the correctionpatterns 14 formed on the transfer belt 3. The detection sensors 15 and16 output analog detection signals indicating the positions of thedetected correction patterns 14 to the engine control unit 113.

The printer controller 111 receives image data transmitted from anexternal apparatus (e.g., a personal computer (PC)) via a network. Theprinter controller 111 transfers the received image data to the enginecontrol unit 113 (an image processing unit 124, which will be describedlater).

The scanner controller 112 receives scanned image data of an originalfrom a scanner (not shown). The scanner controller 112 transfers thereceived scanned image data to the engine control unit 113 (the imageprocessing unit 124).

The engine control unit 113 includes a pattern detecting unit 121, acentral processing unit (CPU) 122, a random access memory (RAM) 123, theimage processing unit 124, and a write control unit 125.

The pattern detecting unit 121 receives the analog detection signalsfrom the detection sensors 15 and 16 and amplifies the analog detectionsignals, converts the amplified analog detection signals into digitaldata, and stores the converted digital data in the RAM 123.

The CPU 122 calculates the positions of the correction patterns 14according to the digital data stored in the RAM 123, calculatesmisalignment amounts from the calculated positions, and then calculatesmisalignment correction amounts based on the calculated misalignmentamounts. The misalignment amounts can be a distortion amount, amagnification error amount in the main-scanning direction, amain-scanning direction misregistration amount, a sub-scanning directionmisregistration amount, and a skew amount of each color. Themisalignment correction amounts can be a distortion correction amount, amain-scanning-direction-magnification correction amount, asub-scanning-direction-registration correction amount, asub-scanning-direction-registration correction amount, and a skewcorrection amount of each color.

When K is set as a reference color, the CPU 122 calculates the numbersof distorted lines of Y, M, and C relative to the reference color Kbased on a resolution of image data and the calculated distortion amountof each color (Y, M, C, K). The CPU 122 determines the number of linesfor a line memory of each color based on the number of distorted linerelative to the reference color. The reference color is a color to beused as the reference for calculation of the distortion amount of colorsother than the reference color. The reference color is assumed to be K(black).

The RAM 123 temporarily stores therein the digital data indicating thepositions of the correction patterns 14 that is fed from the patterndetecting unit 121 via the CPU 122. Meanwhile, a nonvolatile memory canbe used in place of the RAM 123. In this case, the digital dataindicating the positions of the correction patterns 14 is stored in thenonvolatile memory.

The image processing unit 124 performs various image processingaccording to image data that is received by the printer controller 111or that is transmitted from the scanner controller 112 to convert theimage data into image data (e.g., 1-bit binary image data) of eachcolor. In the first embodiment, based on sub-scanning timing signal(K,M,C,Y)_FSYNC_N supplied from the write control unit 125 for eachcolor, the image processing unit 124 transmits image data(K,M,C,Y)_IPDATA_N accompanied by main-scanning gate signal(K,M,C,Y)_IPLGATE_N and sub-scanning gate signal (K,M,C,Y)_IPFGATE_N,which are synchronization signals, to the write control unit 125.

The write control unit 125 includes a write control unit 126 (126K,126M, 126C, and 126Y). The write control units 126K, 126M, 126C, and126Y generates print timing signals that indicate the write controlunits 126K, 126M, 126C, and 126Y when to form latent images of thecorresponding colors on the photosensitive drums 7Y, 7M, 7C, and 7K.According to the generated print timing signals, the write control unit125 receives the image data and performs various write-control imageprocessing of the received image data to convert the image data into LDlight-emission data (K,M,C,Y)_LDDATA. The write control unit 125transmits K_LDDATA, M_LDDATA, C_LDDATA, and Y_LDDATA to the LD controlunit 114K, the LD control unit 114M, the LD control unit 114C, and theLD control unit 114Y, respectively.

The LD control units 114K, 114M, 114C, and 114Y are parts of theexposure unit 9. The LD control units 114K, 114M, 114C, and 114Y receivethe LD light-emission data from the write control unit 125 and outputdrive signals to the exposure unit 9 according to the LD light-emissiondata. The exposure unit 9 controls emission of laser beams LY, LM, LC,and LK toward the photosensitive drums 7Y, 7M, 7C, and 7K according tothe drive signals. As a result, the laser beam LY, LM, LC, and LK formlatent images on the surfaces of the photosensitive drums 7Y, 7M, 7C,and 7K.

How the color copier 60 performs color image forming will be brieflydescribed below. An image that is, for example, received from a personalcomputer (PC) is processed by any one of the printer controller 111 thatperforms processing for printing and the scanner controller 112 thatperforms processing for making a copy of the image. Resultant image datais transferred to the image processing unit 124 of the engine controlunit 113. The image processing unit 124 performs various imageprocessing on the image data, converts the processed image data intoimage data of each color, and transfers the image data to the writecontrol unit 125. The write control unit 125 receives the image data,performs various write image processing on the image data, converts theprocessed image data into LD light-emission data, and causes the LDs toemit light to form a latent image on each of the photosensitive drums7Y, 7M, 7C, and 7K based on the LD light-emission data.

The write control unit 125 will be described in more detail withreference to FIG. 4. FIG. 4 is a block diagram of an exemplaryconfiguration of the write control unit 125. The write control unit 125includes the write control units 126 (126K, 126M, 126C, and 126Y),input-image control units 127 (127K, 127M, 127C, and 127Y), and linememories 128 (128K, 128M, 128C, and 128Y).

The write control unit 126K for K, which is the reference color,includes a write-image processing unit 131K, amisalignment-correction-pattern creating unit 132K, and an LD-dataoutput unit 133K. Each of the write control units 126M, 126C, and 126Yfor M, C, and Y, which are the colors other than the reference color,has a similar configuration to that of the write control unit 126K. Thewrite control unit 126 (126M, 126C, 126Y) includes a write-imageprocessing unit 131 (131M, 131C, 131Y), amisalignment-correction-pattern creating unit 132 (132M, 132C, 132Y),and an LD-data output unit 133 (133M, 133C, 133Y). However, the writecontrol units 126M, 126C, and 126Y additionally include skew-correctionprocessing units 135M, 135C, and 135Y, respectively.

Note that in FIG. 4, sets of the main-scanning gate signal(K,M,C,Y)_IPLGATE_N, the sub-scanning gate signal (K,M,C,Y)_IPFGATE_N,and the image data (K,M,C,Y)_IPDATA_N described above with reference toFIG. 3 are collectively denoted as a write control signal(K,M,C,Y)_IPDATA[7:0]_N for clarity of description.

The input-image control unit 127 receives the write control signal(K,M,C,Y)_IPDATA[7:0]_N from the image processing unit 124. Theinput-image control unit 127 divides the write control signal(K,M,C,Y)_IPDATA[7:0]_N in the sub-scanning direction in such a mannerthat image data (image) is divided into a plurality of groups each ofwhich includes at least one line in the main-scanning direction(hereinafter, “main-scanning line”). The main-scanning lines are storedin the line memory 128. The input-image control unit 127 transfers themain-scanning lines line-by-line to the write control unit 126 whilecausing the line memory 128 to be toggled.

In the first embodiment, the input-image control units 127 (127K, 127M,127C, and 127Y) store the write control signals (K,M,C,Y)_IPDATA[7:0]_Nin the line memories 128 (128K, 128M, 128C, 128Y) based on the number ofdistorted lines that is calculated by the CPU 122. The input-imagecontrol units 127 (127K, 127M, 127C, and 127Y) receive the 1-bit binaryimage data (the write control signal (K,M,C,Y)_IPDATA[7:0]_N) from theimage processing unit 124, and transfer the 1-bit binary image data tothe write control units 126 (126K, 126M, 126C, and 126Y). Note that theimage data transferred to the write control units 126 is not limited tosuch 1-bit binary image data. For example, binary image data can beconverted into 4-bit image data that represents image with densityvalues ranging from 0 (for white pixel) to 15 (for black pixel) that isthen transferred to the write control units 126 (126K, 126M, 126C, and126Y).

The line memories 128K, 128M, 128C, and 128Y receive the write controlsignal (K,M,C,Y)_IPDATA[7:0]_N from the image processing unit 124 andsequentially store the write control signals therein.

The write-image processing units 131K, 131M, 131C, and 131Y receive thewrite control signals (K,M,C,Y)_IPDATA[7:0]_N transferred from theinput-image control units 127K, 127M, 127C, and 127Y (or theskew-correction processing units 135M, 135C, and 135Y, which will bedescribed later), perform various write-control image processing of thewrite control signals, and transfer the processed write control signalsto the LD-data output units 133K, 133M, 133C, and 133Y.

The misalignment-correction-pattern creating units 132K, 132M, 132C, and132Y create the correction patterns 14 that are to be transferred ontothe transfer belt 3. The misalignment correction amounts for use incorrection of color-to-color misalignment on the transfer belt 3 arecalculated by using the correction patterns 14.

The LD-data output units 133 (133K, 133M, 133C, and 133Y) convert thewrite control signals (K,M,C,Y)_IPDATA[7:0]_N transferred from thewrite-image processing units 131 (131K, 131M, 131C, and 131Y) into theLD light-emission data (K,M,C,Y)_LDDATA. The LD-data output units 133(133K, 133M, 133C, and 133Y) transmit correction-related data (LDDATA)to the LD control units 114 (114K, 114M, 114C, and 114Y). Thecorrection-related data is created on the main-scanning directionmisregistration amounts and the sub-scanning direction misregistrationamounts calculated by the CPU 122 for correction of improper write-starttiming of laser beam emission. The LD-data output units 133 (133K, 133M,133C, and 133Y) also transmit data (LDDATA) for adjusting imagefrequency to the LD control units 114 (114K, 114M, 114C, and 114Y). Thisdata is created based on the main-scanning-direction magnification erroramounts calculated by the CPU 122 for correction of magnification errorin the main-scanning direction. The LD-data output units 133 (133K,133M, 133C, and 133Y) further transmit data (LDDATA) for forming thecorrection patterns 14 generated by the misalignment-correction-patterncreating units 132 (132K, 132M, 132C, and 132Y) on the transfer belt 3to the LD control units 114 (114K, 114M, 114C, and 114Y). This data iscreated based on the main-scanning-direction magnification error amountscalculated by the CPU 122 for correction of magnification error in themain-scanning direction.

The LD-data output unit 133 of each color includes a device, such as aclock generator that uses a voltage controlled oscillator (VCO), capableof setting a frequency finely. The LD-data output unit 133 periodicallyforms (outputs) an image based on the image data (the write controlsignal (K,M,C,Y)_IPDATA[7:0]_N) of each color on the transfer belt 3according to a preset output frequency of the clock generator.

The skew-correction processing units 135M, 135C, and 135Y perform skewcorrection of the write control signals (M,C,Y)_IPDATA[7:0]_N based onthe reference color, K. Specifically, the skew-correction processingunit 135 divides pixels that belong to one main-scanning line of theimage data stored in the line memory 128 into pixel blocks, shifts imagedata of one of the pixel blocks in the sub-scanning direction against adirection of skew, and transfers the image data to the write-imageprocessing unit 131. Hence, the skew that can otherwise occur during theprocess of forming a toner image can be corrected. How the write controlunit 126 performs an image writing process will be described in detail.

A process for writing a K-image will be described with reference to FIG.4. The image processing unit 124 transmits image data K_IPDATA[7:0]_N tothe input-image control unit 127K. The input-image control unit 127Ktransmits the image data to the input-image control unit 127K whiletemporarily storing the image data in the line memory 128K. In the writecontrol unit 126K, the write-image processing unit 131K receives theimage data from the input-image control unit 127K and transmits theimage data to the LD-data output unit 133K. Based on the image data, theLD-data output unit 133K generates the light-emission data K_LDDATA forK and transmits the generated data to the LD control unit 114K.

A process for writing M-, C-, and Y-images will be described withreference to FIG. 4. The image processing unit 124 transmits image data(M,C,Y)_IPDATA[7:0]_N to the input-image control units 127M, 127C, and127Y. The input-image control units 127M, 127C, and 127Y temporarilystore the image data in the line memories 128M, 128C, and 128Y toperform skew correction according to skew correction amounts stored inthe RAM 123. The skew-correction processing units 135M, 135C, and 135Ycorrect skew of the temporarily-stored image data according to the skewcorrection amounts, and transmits the corrected image data to thewrite-image processing units 131M, 131C, and 131Y, respectively. As inthe case of the process for K, the LD-data output units 133M, 133C, and133Y receive the image data from the write-image processing units 131M,131C, and 131Y, generate the light-emission data (M,C,Y)_LDDATA, andtransmits the generated light-emission data (M,C,Y)_LDDATA to the LDcontrol units 114M, 114C, and 114Y, respectively. The skew correctionamounts will be described below.

Meanwhile, the correction patterns 14 are formed in a manner similar tothat described above; however, the correction patterns 14 are formedbased on pattern image data for K-, M-, C-, and Y-correction patternsthat are transmitted from the misalignment-correction-pattern creatingunits 132K, 132M, 132C, and 132Y and received by the LD-data outputunits 133K, 133M, 133C, and 133Y.

As described above, occurrence of color misalignment is a criticalproblem when forming a full-color image by superimposing K-, M-, C-, andY-toner images on one another. A process of correcting colormisalignment will be described with reference to FIG. 5. FIG. 5 is aflowchart for explaining correction of the color misalignment. Themisalignment correction will be described below on an assumption thatthe reference color is K. The reference color functions as a referencein color misalignment correction. Color-to-color misalignment iscorrected by adjusting the other colors to the reference color.

When the write control unit 125 is commanded to start misalignmentcorrection by the CPU 122, the write control unit 125 forms thecorrection patterns 14 on the transfer belt 3 (Step S11). The correctionpatterns 14 are created by the misalignment-correction-pattern creatingunits 132K, 132M, 132C, and 132Y in the write control units 126K, 126M,126C, and 126Y depicted in FIG. 4. The detection sensors 15 and 16detect positions of the correction patterns 14, and output detectionsignals that indicate the positions of the correction patterns 14 to thepattern detecting unit 121 (Step S12).

The pattern detecting unit 121 receives the detection signals, convertsthem into digital data. The CPU 122 calculates amain-scanning-direction-magnification correction amount, amain-scanning-direction-registration correction amount, and asub-scanning-direction-registration correction amount of each colorrelative to the reference color (K) based on the positions of thecorrection patterns 14 according to the digital data (Step S13). The CPU122 also calculates a skew correction amount of each color relative tothe reference color (K) (Step S14). The CPU 122 then calculatescorrecting directions and dividing positions in the main-scanningdirection for skew correction (Step S15).

The CPU 122 stores information that includes information about themain-scanning-direction magnification correction amounts, themain-scanning-direction registration correction amount, thesub-scanning-direction registration correction amount, the skewcorrection amounts, and the correcting directions and the dividingpositions in the main-scanning direction for skew correction in the RAM123 (or in a nonvolatile memory) (Step S16). Then, the process controlends. The correction amounts stored in the RAM 123 will be used ascorrection amounts for use in printing before completion of thisprocedure that would be performed to correct misalignment that occursnext time.

After the main-scanning-direction magnification correction amounts, themain-scanning-direction registration correction amounts, thesub-scanning-direction registration correction amounts, the skewcorrection amounts, and the correcting directions in the main-scanningdirection and the dividing positions for skew correction of each colorof M, C, and Y have been stored in the RAM 123 (or in the nonvolatilememory) as described above, a printing process is performed. FIG. 6 is aflowchart for explaining how printing is performed.

Upon receiving a print request from the CPU 122, the write control unit125 sets a pixel clock frequency for each color of K, M, C, and Y basedon the main-scanning-direction magnification correction amounts (StepS31). The write control unit 125 sets a delay amount in themain-scanning direction of each color (Step S32), and sets a delayamount in the sub-scanning direction of each color (Step S33).

The write control unit 125 sets a skew correction amount of each colorof M, C, and Y relative to the reference color (K) based on the skewcorrection amount and information about the number of levels of eachcolor (Step S34). The write control unit 125 starts printing whileperforming image correction for each color of K, M, C, and Y based onthe set pixel clock frequencies, the delay amounts in the main-scanningdirection, and the delay amount in the sub-scanning direction, and theskew correction amounts (Step S35). Then, the process control ends.

The main-scanning-direction misalignment is corrected by correcting themain-scanning-direction magnification and write-start timing in themain-scanning direction. The main-scanning-direction magnification errorcan be corrected by adjusting a picture frequency based on themain-scanning-direction magnification correction amount of each colorcalculated by the write control unit 125. The write control unit 125includes a device, such as a clock generator that uses a voltagecontrolled oscillator (VCO), capable of setting a frequency finely. Acounter in the main-scanning direction is triggered by a synchronizationdetection signal of each color. The write-start timing in themain-scanning direction is adjusted depending on a position on an outputof the counter at which the LD starts output of data.

The sub-scanning-direction misalignment is corrected by adjustingwrite-start timing in the sub-scanning direction. FIG. 7 is a timingchart for explaining write timing correction in the sub-scanningdirection performed by the write control unit 125. The write controlunit 125 counts the number of lines in response to a start signalSTTRIG_N that serves as a reference and outputs a sub-scanning timingsignal (Y,M,C,K)_FSYNC_N to the image processing unit 124.

The sub-scanning timing signal (Y,M,C,K)_FSYNC_N triggers the imageprocessing unit 124 to output the sub-scanning timing signal(Y,M,C,K)_FSYNC_N to the write control unit 125 and transfer the imagedata K_IPDATA[7:0]_N to the write control unit 125. The write controlunits 126K, 126M, 126C, and 126Y transmit the LD light-emission data(K,M,C,Y)_LDDATA to the LD control units 114K, 114M, 114C, and 114Y.

The sub-scanning-direction misregistration is corrected by adjustingsub-scanning delay amounts (Y,M,C,K)_mfcntld relative to the startsignal according to the calculated misregistration amounts. It isgeneral to perform alignment in the sub-scanning direction by adjustingtimings (Y,M,C,K)_mfcntld while taking the sub-scanning delay amount ofeach color (M, C, and Y) relative to the reference color K intoconsideration.

How the color copier 60 calculates the misalignment amounts and correctsthe misalignment will be described below. The detection sensors 15 and16 detect the positions of the correction patterns 14 and outputdetection signals. The pattern detecting unit 121 converts the detectionsignals from analog data into digital data, which then undergoessampling. The sampled digital data is stored in the RAM 123. After theprocedure related to the detection of the correction patterns 14 iscompleted, the CPU 122 performs computations for calculations of thevarious misalignment amounts (the main-scanning-direction-magnificationerror amounts, the main-scanning direction misregistration amounts, thesub-scanning direction misregistration amounts, and the skew amounts).The CPU then calculates the correction amounts (themain-scanning-direction-magnification correction amounts, themain-scanning direction correction amounts, the sub-scanning directioncorrection amounts, and the skew correction amounts) of the misalignmentcomponents from the misalignment amounts.

How to calculate the skew amounts and the skew correction amounts foruse in the skew correction will be described. FIG. 8 is a schematicdiagram of exemplary correction patterns formed on the transfer belt 3.FIG. 9 is a schematic diagram for explaining a method of calculating askew amount. FIG. 9 depicts an example of the method of calculating askew amount of each color by using K as the reference color.

The CPU 122 calculates a skew amount of each color (M, C, Y) relative tothe reference color K. A state in which, as depicted in FIG. 9, rightsections of C-correction patterns (C11 and C21) of the correctionpatterns 14 are shifted downward as compared to those in a normal statewill be described as an example. The detection sensor 15 on the leftside in FIG. 9 detects positions of some (K11 and C11) of left sectionsof the correction patterns 14 and calculates KC_L, which is a distancebetween the left section of the K-correction pattern and that of theC-correction pattern based on a relationship between the detectedpositions. The detection sensor 16 on the right side in FIG. 9 detectspositions of some (K21 and C21) of right sections of the correctionpatterns 14 and calculates KC_R, which is a distance between the rightsection of the K-correction pattern and that of the C-correction patternbased on a relationship between the detected positions. Hence, KC_Skew,which is a C-skew amount relative to K, is calculated by using Equation(1):KC_Skew=KC _(—) R−KC _(—) L  (1)

KM_Skew, which is an M-skew amount relative to K, and KY_Skew, which isa Y-skew amount relative to K, can be similarly calculated by detectingthe positions of the correction patterns 14 and by using Equations (2)and (3), respectively:KM_Skew=KM _(—) R−KM _(—) L  (2)KY_Skew=KY _(—) R−KY _(—) L  (3)

KC_Skew, which is the C-skew amount, KM_Skew, which is the M-skewamount, and KY_Skew, which is the Y-skew amount, relative to K can becalculated as described above.

Calculation of a skew correction amount based on the skew amounts willbe described below in detail by way of an example. FIG. 10 is a table ofexemplary skew amounts relative to the reference color K for aresolution of 600 dots per inch (dpi) in the sub-scanning direction. Itis assumed that the skew amount of each color is calculated as depictedin FIG. 10 by using Equations (1) to (3). More specifically, it isassumed that the skew amount of each color are such that the M-skewamount is −110 micromillimeters, the C-skew amount is −130micromillimeters, and the Y-skew amount is 30 micromillimeters. Becausethe resolution in the sub-scanning direction is 600 dpi, shifting by oneline corresponds in distance to a displacement of 42.3 micromillimetersthat is obtained by dividing 25400 micromillimeters by 600. Hence, thescrew correction amount can be calculated by dividing the skew amount ofeach color by the displacement distance for one-line shifting, roundingoff the quotient to the number of decimal, and inverting the sign of thevalue. FIG. 11 is a table of exemplary skew correction amounts obtainedfrom the skew amounts of FIG. 10. As depicted in FIG. 11, the M-skewcorrection amount is +3 lines, the C-skew correction amount is +3 lines,and the Y-skew correction amount is −1 line.

An example of the skew correction method (method of calculating the skewcorrection amounts) will be described with reference to FIGS. 12 to 17.FIG. 12 is a schematic diagram of an input image of eight lines of imagedata. One line of the image data corresponds to image data stored in asingle line memory. FIG. 13 is a schematic diagram of an output imagethat is obtained by outputting image data of the input image depicted inFIG. 12 as LD light-emission data without performing the skewcorrection. When the image data is output as the LD light-emission datawithout being subjected to the skew correction as in this example, skewof a scanning beam causes a right side of an output image on a sheet tobe deviated upward by three lines as compared to the input imagedepicted in FIG. 12. In other words, the skew correction amount for theimage depicted in FIG. 13 is three lines.

When, as in this case, a right side of an output image is deviatedupward by three lines, pixels belonging to one line of the image data inthe main-scanning direction are divided into {(the number of lines ofthe skew correction amount)+1} equal blocks. More specifically, in thisexample, 4800 pixels are divided into four equal blocks as depicted inFIG. 14. Each of the positions (dividing position) where the pixels onthe line in the main scanning direction are divided is denoted as a“shift position”, and each of regions that are defined by dividing atthe shift positions on the line in the main-scanning direction isdenoted as a shift region.

As depicted in FIG. 15, the skew-correction processing units 135 (135M,135C, and 135Y) causes the pixels divided at the shift positions toshift such that a shift region is shifted downward by one line than aleft-neighboring shift region for each of the shift regions. Thisdownward shift is performed in order to compensate for the skew in thesub-scanning direction. In this manner, the skew-correction processingunits 135 (135M, 135C, and 135Y) correct the skew of the output image onthe sheet as depicted in FIG. 16.

More specifically, a portion of the image data is stored in each of theline memories 128M, 128C, and 128Y in a sequential manner. That portionof the image data is read out from the line memory 128 for each of theshift regions defined by the dividing at the shift positions. Byselectively changing the line memory 128, it is possible to obtain theoutput image depicted in FIG. 16.

Hence, as depicted in FIG. 17, the skew-correction processing unit 135calculates shift positions and shift directions (+ve or −ve) in thesub-scanning direction at the shift positions based on the skewcorrection amounts calculated by the CPU 122. The shift position is anaddress of the corresponding line memory. The shift positions and theshift directions are referred to as shift correction information. Theskew-correction processing unit 135 causes the pixels (shift regions)divided in the main-scanning direction at the shift positions to shiftin the shift directions, thereby performing color-to-color skewcorrection. FIG. 17 is a table of exemplary shift positions and shiftdirections calculated by the skew-correction processing unit 135.

Another example of the skew correction method performed by theskew-correction processing unit 135 will be described with reference toFIGS. 18 to 23. FIGS. 18 to 23 are schematic diagrams for explaining theother example of the skew correction method. FIG. 18 is a schematicdiagram depicting eight lines of image data. FIG. 19 is an output imagethat is obtained by outputting image data pertaining to an input imagedepicted in FIG. 18 as LD light-emission data without performing theskew correction. When the image data is output as the LD light-emissiondata without being subjected to the skew correction, skew of a scanningbeam causes a right side of an output image on a sheet to be shifteddownward by one line as compared with the input image depicted in FIG.18. In other words, the skew correction amount is one line. This skewcan also be corrected by performing similar operations as describedabove with reference to FIGS. 12 to 17.

More specifically, when a right side of an output of image data isundesirably deviated downward by one line, the skew-correctionprocessing units 135M, 135C, and 135Y divide 4800 pixels that belong toone line of the image data in the main-scanning direction into equalblocks. More specifically, the skew-correction processing unit 135divides the pixels into {(the number of lines of the skew correctionamount)+1} equal blocks. It is assumed that the pixels are divided intotwo equal blocks as depicted in FIG. 20. As depicted in FIG. 21, theskew-correction processing units 135M, 135C, and 135Y cause the pixelsdivided at the shift position to shift such that a shift region isshifted upward by one line than a left-neighboring shift region for eachof the shift regions. In this manner, the skew-correction processingunits 135M, 135C, and 135Y correct the skew of the output image on thesheet as depicted in FIG. 22.

More specifically, a portion of the image data is stored in each of theline memories 128M, 128C, and 128Y in a sequential manner. That portionof the image data is read out from the line memory 128 for each of theshift regions defined by the dividing at the shift positions. Byselectively changing the line memory 128, it is possible to obtain theoutput image depicted in FIG. 16 or FIG. 22.

Hence, as depicted in FIG. 17 and FIG. 23, the skew-correctionprocessing unit 135 calculates shift position and shift direction (+veor −ve) in the sub-scanning direction at the shift positions based onthe skew correction amounts calculated by the CPU 122. The shiftposition is an address of the corresponding line memory. The shiftpositions and the shift directions are referred to as shift correctioninformation. The skew-correction processing unit 135 causes the pixels(shift region) divided in the main-scanning direction at the shiftposition to shift in the shift directions, thereby performingcolor-to-color skew correction. Information including the address of theshift position in the main-scanning direction and the shift direction(+ve or −ve) in the sub-scanning direction at the shift position isdenoted as shift correction information.

The shift correction information is stored in the RAM 123. The shiftcorrection information is obtained based on the skew correction amountscalculated by the CPU 122. The skew-correction processing unit 135retrieves the shift correction information from the RAM 123 for eachskew correction and performs the skew correction based on the shiftcorrection information. The skew-correction processing unit 135 updatesthe shift correction information when a new piece of the shiftcorrection information is obtained.

Assume that, for example, image data represents 4800 pixels in themain-scanning direction as depicted in FIG. 14. Because the pixels onthe right end are deviated upward by three lines relative to the pixelson the left end, the skew-correction processing unit 135 divides the4800 pixels that belong to one line of image data in the main-scanningdirection into four equal blocks. More specifically, the skew-correctionprocessing unit 135 divides the pixels into a shift region of the 1st to1200th pixels, that of the 1201st to 2400th pixels, that of the 2401stto 3600th pixels, and that of the 3601st to 4800th pixels, these shiftregions are denoted as a first block, a second block, a third block, anda fourth block, respectively.

The numbers 1 to 8 in FIG. 14 indicate the ordinal numbers of the eightlines. As depicted in FIG. 15, for the 1st to 1200th pixels of the firstline, the skew-correction processing unit 135 outputs the first block ofthe image data in the line memory for the first line and outputs whitepixels for the 1201st to 4800th pixels. For the 1st to 1200th pixels ofthe second line, the skew-correction processing unit 135 outputs thefirst block of the image data in the line memory for the second line.For the 1201st to 2400th pixels of the second line, the skew-correctionprocessing unit 135 outputs the second block of the image data in theline memory for the first line, and outputs white pixels for the 2401stto 4800th pixels of the second line. By repeating an image-data outputprocess in this manner, the skew-correction processing unit 135 correctsthe skew of an output image on a sheet as depicted in FIG. 16.

FIGS. 24 and 25 are timing charts of read/write timings for reading andwriting performed by the write control unit 125 in the sub-scanningdirection. It is assumed that because K is the reference color, dividingfor the K-image is not performed. A skew correction amount for each of Mand C is three dots (lines), and a skew correction amount for Y is onedot (line). Accordingly, for each of M and C, three shift regions ofequally-divided four shift regions are shifted in the shift direction,while for Y, one shift region of equally-divided two shift regions isshifted in the shift direction.

The input-image control unit 127 starts a printing operation after asub-scanning delay (K,M,C,Y)_mfcntld from the start signal STTRIG_N.When printing is started, the input-image control unit 127 stores imagedata in line memories K-1, M-1, C-1, and Y-1.

Subsequently, the input-image control unit 127 stores image data in linememories K-2, M-2, C-2, and Y-2, and simultaneously reads from the linememories K-1, M-1, C-1, and Y-1 the image data stored therein. The writecontrol unit 126K causes all pixels of the line memory K-1 to be outputto the K-LD light-emission data K_LDDATA. The write control unit 126Mcauses pixels of the first block of the equally-divided four shiftregions of the line memory M-1 to be output to M-LD light-emission dataM_LDDATA. The write control unit 126C causes pixels of the first blockof the equally-divided four shift regions of the line memory C-1 to beoutput to C-LD light-emission data C_LDDATA. The write control unit 126Ycauses pixels of the first block of the equally-divided two shiftregions of the line memory Y-1 to be output to Y-LD light-emission dataY_LDDATA.

The input-image control units 127K, 127M, 127C, and 127Y store imagedata in line memories K-3, M-3, C-3, and Y-3, and simultaneously readsfrom the line memories K-2, M-1, M-2, C-1, C-2, Y-1, and Y-2 the imagedata stored therein. The write control unit 126K causes all pixels ofthe line memory K-2 to be output to the K-LD light-emission dataK_LDDATA. The write control unit 126M causes pixels of the second blockof the line memory M-1 and pixels of the first block of the line memoryM-2 to be output to the M-LD light-emission data M_LDDATA. The writecontrol unit 126C causes pixels of the second block of the line memoryC-1 and pixels of the first block of the line memory C-2 to be output tothe C-LD light-emission data C_LDDATA. The write control unit 126Ycauses pixels of the second block of the line memory Y-1 and pixels ofthe first block of the line memory Y-2 to be output to the Y-LDlight-emission data Y_LDDATA.

The input-image control units 127K, 127M, 127C, and 127Y store imagedata in line memories K-4, M-4, C-4, and Y-1, and simultaneously readsfrom the line memories K-1, M-1, M-2, M-3, C-1, C-2, C-3, Y-2, and Y-3the image data stored therein. The write control unit 126K causes allpixels of the line memory K-3 to be output to the K-LD light-emissiondata K_LDDATA. The write control unit 126M causes pixels of the thirdblock of the line memory M-1, pixels of the second block of the linememory M-2, and pixels of the first block of the line memory M-3 to beoutput to the M-LD light-emission data M_LDDATA. The write control unit126C causes pixels of the third block of the line memory C-1, pixels ofthe second block of the line memory C-2, and pixels of the first blockof the line memory C-3 to be output to the C-LD light-emission dataC_LDDATA. The write control unit 126Y causes pixels of the second blockof the line memory Y-2 and pixels of the first block of the line memoryY-3 to be output to the Y-LD light-emission data Y_LDDATA.

The input-image control units 127K, 127M, 127C, and 127Y store imagedata in line memories K-5, M-5, C-5, and Y-2, and simultaneously readsfrom the line memories K-2, M-1, M-2, M-3, M-4, C-1, C-2, C-3, C-4, Y-1,and Y-3 the image data stored therein. The write control unit 126Kcauses all pixels of the line memory K-4 to be output to the K-LDlight-emission data K_LDDATA. The write control unit 126M causes pixelsof the fourth block of the line memory M-1, pixels of the third block ofthe line memory M-2, pixels of the second block of the line memory M-3,and pixels of the first block of the line memory M-4 to be output to theM-LD light-emission data M_LDDATA. The write control unit 126C causespixels of the fourth block of the line memory C-1, pixels of the thirdblock of the line memory C-2, pixels of the second block of the linememory C-3, and pixels of the first block of the line memory C-4 to beoutput to the C-LD light-emission data C_LDDATA. The write control unit126Y causes pixels of the second block of the line memory Y-3 and pixelsof the first block of the line memory Y-4 to be output to the Y-LDlight-emission data Y_LDDATA. The above procedure is repeatedlyperformed, and printing of the skew-corrected image data is performed.

In the skew correction described above, the pixels that belong to oneline of image data in the main-scanning direction are divided into aplurality of blocks. Dividing one line of image data in a plurality ofblocks, however, can change relationship between adjacent pixels on eachof the shift positions leading to a local color density change at theshift position, i.e., density shift. This density shift is particularlynoticeable in an image processed by a digital halftoning method such asdither. Because local color density change occurs at a shift position ina dithered image at regular intervals in the sub-scanning direction,density shift is particularly noticeable in the dithered image.

Why performing the skew correction on a dithered image data can resultin density shift will be described below. A color MFP such as a colorlaser printer includes different dither matrices for smooth tonetransition. The dither matrices differ from one another for differentcolors, for each of a photo mode and a character mode, for differentclasses of the number of bits, for different levels of resolutions, andthe like. The dither matrices differ from one another in size and shapein many cases.

Dithering is a method of converting a multi-level image into a binaryimage. This binarization is performed by applying a matrix, what iscalled dither matrix, of threshold values of N×M pixels (both N and Mare positive integers) to an original, multi-level image. Each pixel(dither matrix size) is so small that the obtained binary image isperceived as being a gray-level image. Thus, dithering is a technique ofsimulating multiple tones by using binary values. A multi-level dithermethod of obtaining a multi-level image by setting the number of levelsof resultant dithered images to 3 to 16 rather than 2 can also be used.A binary image will be described below as an example; however, thepresent invention is applicable to multiple-level images as well.

In electrophotographic recording, because the diameter of a laser beamis greater than the size of a pixel, a toner area coverage of each pixelis greater than the size of the pixel on an actually recorded image(toner image on printing paper). When the shifting for skew correction(hereinafter, “corrective shifting”) is performed, an area where toneroverlaps (hereinafter, “toner-overlapping area”) can increase ordecrease at a shift position. Accordingly, a toner area coverage candecrease or increase at the shift position.

For example, in a case where a toner area coverage increases (i.e., atoner-overlapping area of pixels of image data decreases) by correctiveshifting, a local color density on or in the vicinity of a shiftposition increases. In contrast, in a case where a toner area coveragedecreases (i.e., toner-overlapping area of pixels of image dataincreases) by corrective shifting, a local color density on or in thevicinity of a shift position decreases. Because this change in tonerarea coverage occurs only at the shift position, an image in thevicinity of the shift position can be degraded by the correctiveshifting. In particular, in a digitally-halftoned image such as adithered image, when the toner area coverage is changed at a number ofpositions, the corrective shifting can result in banding noise extendingin the sub-scanning direction.

An exemplary case where corrective shifting results in an increase ordecrease of a toner area coverage will be described specifically. FIG.26 is a schematic explanatory diagram of an example of a pixel of whichtoner area coverage is changed by corrective shifting.

The corrective shifting in the sub-scanning direction is performed on aline-by-line basis. Accordingly, the corrective shifting causes one oftwo adjacent pixels, which are adjacent to each other with the shiftposition therebetween, to be shifted in the sub-scanning direction byone pixel. Hence, relationship between the adjacent pixels with theshift position therebetween can be changed by the corrective shifting inthe sub-scanning direction. FIG. 26 depicts that before correctiveshifting, both a pixel P1 and its neighboring pixel P2 are black pixels.However, the pixel P2 adjacent to the pixel P1 is changed to a whitepixel after the corrective shifting. When such an image in which a pixeladjacent to a certain pixel is changed is output, as depicted in abottom diagram of FIG. 26 depicting pixels after the correctiveshifting, a toner area coverage is changed by an area depicted as across-hatched area.

Assume that, for example, the toner area coverage within the pixel P1is 1. After corrective shifting downward in the sub-scanning directionis performed at the shift position, the pixel P1 and the pixel P2 do nothave toner-overlapping area, which has been present before thecorrective shifting, therebetween any more. As a result, the toner areacoverage increases by 0.09. When such an increase in toner area coverageoccurs at regular intervals in the sub-scanning direction on a shiftposition, black banding noise that degrades image quality can appear onan output image.

In contrast, although not depicted, when a toner-overlapping area iscreated by corrective shifting, the toner area coverage decreases by0.09. When such a decrease in the toner coverage area occurs at regularintervals in the sub-scanning direction on a shift position, whitebanding noise that degrades image quality can appear on an output image.

The corrective shifting can reduce misalignment due to skew or curve;however, the corrective shifting can disadvantageously result in bandingnoise on an output image of digitally-halftoned image data, inparticular. To this end, the skew-correction processing units 135M,135C, and 135Y of the first embodiment not only perform the correctiveshifting but also compensate a change in density resulting from thecorrective shifting. The skew-correction processing units 135M, 135C,and 135Y will be described in detail below.

FIG. 27 is a detailed block diagram of the skew-correction processingunit 135. The skew-correction processing unit 135 can be any one of theskew-correction processing units 135M, 135C, and 135Y. Theskew-correction processing unit 135 includes a data selector 1351, askew-output control unit 1352, and a noise-correction processing unit1353.

The skew-output control unit 1352 retrieves the shift correctioninformation (information about the shift position and the shiftdirection) from the RAM 123, and outputs a selection signal forselecting image data to be output based on the shift correctioninformation. The image data to be output is selected from image datastored in the line memory 128M by designating one of the lines of theline memory 128M. The skew-output control unit 1352 outputs the shiftcorrection information to the noise-correction processing unit 1353.

The data selector 1351 selects the image data of the designated line tobe output from the image data having been read from the line memory 128Mby the input-image control unit 127M based on the selection signaloutput from the skew-output control unit 1352. The data selector 1351outputs the selected image data to the noise-correction processing unit1353. More specifically, in the first embodiment, the data selector 1351outputs, in addition to the image data of the designated line, imagedata of the line immediately above the designated line and that of theline immediately below the designated line (image data pertaining tothese three lines in total) to the noise-correction processing unit1353. In the first embodiment, the three lines×two pixels of image datais output to the noise-correction processing unit 1353; however, theimage data to be output to the noise-correction processing unit 1353 isnot limited thereto. For example, image data corresponding to more thanthree lines can be output to the noise-correction processing unit 1353depending on processing performed by the noise-correction processingunit 1353.

The noise-correction processing unit 1353 receives the shift correctioninformation from the skew-output control unit 1352 and the image datafrom the data selector 1351. The noise-correction processing unit 1353extracts, from the image data, one or more pixels at a position wherenoise is determined to occur in the vicinity of the shift position. Thenoise-correction processing unit 1353 corrects a color density of imagedata of the extracted pixel to prevent noise, and outputs thecolor-density-corrected image data to the write-image processing unit131.

The configuration and process control of the noise-correction processingunit 1353 according to the first embodiment will be described withreference to FIGS. 28 and 29. FIG. 28 is a block diagram of thenoise-correction processing unit 1353 according to the first embodiment.FIG. 29 is a flowchart depicting how the noise-correction processingunit 1353 performs density correction and phase correction.

The noise-correction processing unit 1353 includes a noise-occurrencedetermining unit 1501, a correction-target-pixel selecting unit 1502, adensity-distribution determining unit 1503, a density correcting unit1504, a phase correcting unit 1505, and a corrected-pixel-data outputunit 1506. Each of the noise-correction processing unit for Y, M, and Chas the similar configuration with that of the noise-correctionprocessing unit 1353 depicted in FIG. 28.

The noise-occurrence determining unit 1501 receives the shift correctioninformation from the skew-output control unit 1352, and determineswhether a pixel of interest on the shift position is a noise-inducingpixel that leads to local color density change because of a change inrelationship with an adjacent pixel of the noise-inducing pixel (StepS1511).

FIG. 30 is a schematic diagram for explaining how a pixel of interestbecomes a noise-inducing pixel by way of an example. Because a rightsection of the image is shifted downward in this example, a colordensity of image data of a pixel-of-interest 1601 and apixel-of-interest 1602 changes to 0 which is a density value for whitepixels. Because a toner area coverage indicated as cross-hatched areasin FIG. 30 hence increases, the noise-occurrence determining unit 1501determines that the pixel-of-interest 1601 and the pixel-of-interest1602 are noise-inducing pixels.

In the first embodiment, the noise-occurrence determining unit 1501determines a pixel of interest as being a noise-inducing pixel when apixel arrangement and a shift direction of pixels in the vicinity of theshift position match a preset pixel arrangement pattern. This pixelarrangement pattern is such a pattern that when corrective shifting ofimage data that matches the pixel arrangement pattern is performed, atoner area coverage increases or decreases, resulting in a localincrease or decrease of density on an output of the image data. FIGS. 31and 32 are schematic diagrams of examples of pixels in the vicinity of ashift position. When corrective shifting as depicted in FIG. 30 isperformed, the noise-occurrence determining unit 1501 determines whethera pixel of interest is a noise-inducing pixel by comparing a pixelarrangement 1701 of three lines×two pixels depicted in FIG. 31 and apixel arrangement 1702 of three lines×two pixels depicted in FIG. 32with a preset pixel arrangement pattern.

In the first embodiment, the noise-occurrence determining unit 1501determines whether a pixel of interest is a noise-inducing pixel byusing image data that has been shifted by the input-image control unit127; however, other data can be used in this determination. For example,whether a pixel of interest is a noise-inducing pixel can be determinedby using not-yet-shifted image data. In a case where the binary imagedata supplied to the input-image control unit 127 from the imageprocessing unit 124 is converted into 4-bit image data that indicates acolor density as a value ranging from 0 to 15, the determination can bemade by increasing the number of the pixel arrangement patterns.Alternatively, the determination can be made by using a density valuerepresented by higher-order bits of the 4-bit image data. This permitsreduction in the number of bits of image data to be input to thenoise-occurrence determining unit 1501.

FIGS. 33 and 34 are schematic diagrams for explaining pixels in thevicinity of a shift position before and after corrective shifting. Pixelarrangements 3301 to 3304 of FIG. 33 and pixel arrangements 3401 and3404 of FIG. 34 are pixels (three lines×two pixels) in the vicinity ofthe shift position before the corrective shifting. Pixel arrangements3305 to 3308 of FIG. 33 and pixel arrangements 3405 and 3408 of FIG. 34are pixels (three lines×two pixels) in the vicinity of the shiftposition after the corrective shifting. A pixel B is a pixel ofinterest. Because a pixel adjacent to the pixel B is changed from apixel C to a pixel A by the corrective shifting, a color density of thepixel adjacent to the pixel B changes. Hence, the noise-occurrencedetermining unit 1501 determines that the pixel B is a noise-inducingpixel.

When the noise-occurrence determining unit 1501 determines that a pixelof interest is a noise-inducing pixel (Yes at Step S1512), thecorrection-target-pixel selecting unit 1502 determines any one of thepixel of interest and a pixel in the vicinity of the pixel of interestthat increases or decreases a color density as a pixel for which densitycorrection of image data is to be performed (hereinafter, “correctiontarget pixel”) (Step S1513). FIGS. 35 and 36 are schematic diagrams ofexamples of correction target pixels that are determined based on thepixels in the vicinity of the shift position.

Assume that, for example, a pixel of interest (pixel having thecross-hatched area) in the pixel arrangement 1701 in the vicinity of theshift position of FIG. 31 is determined as being a noise-inducing pixel.In this case, as depicted in FIG. 35, the correction-target-pixelselecting unit 1502 selects, as the correction target pixel, a pixel2001 of which toner area coverage has been changed from among the pixelsin the pixel arrangement 1701 in the vicinity of the shift position. Incontrast, assume that a pixel of interest (pixel having thecross-hatched area) in the pixel arrangement 1702 in the vicinity of theshift position in FIG. 32 is determined as being a noise-inducing pixel.In this case, as depicted in FIG. 36, the correction-target-pixelselecting unit 1502 selects, as the correction target pixel, a pixel2002 of which toner area coverage has been changed from among the pixelsin the pixel arrangement 1702 in the vicinity of the shift position.

More specifically, when image data of the pixel arrangement 3304 of FIG.33 is output without being subjected to corrective shifting, atoner-overlapping area of the pixel B (black pixel), which is the pixelof interest, and the pixel C (black pixel) does not appear as anadditional toner area coverage. On the other hand, when the image dataof the pixel arrangement 3304 is subjected to corrective shifting andoutput as image data of the pixel arrangement 3308 of FIG. 33, the areawhere the pixel B and the pixel C have overlapped each other before thecorrective shifting changes to a toner-overlapping area of the pixel Band a pixel A (white pixel). Accordingly, a toner area coverageincreases by an amount of the area where the pixel B and the pixel Chave overlapped each other before the corrective shifting. This isbecause, as described above, a laser beam spot-size is larger than asingle pixel area. Hence, the correction-target-pixel selecting unit1502 selects the pixel B as the correction target pixel from among thepixels in the pixel arrangement 3308 in the vicinity of the shiftposition of FIG. 33.

When image data of the pixel arrangement 3302 of FIG. 33 is outputwithout being subjected to corrective shifting, a toner-overlapping areaof the pixel B (black pixel), which is the pixel of interest, and thepixel C (white pixel) appears as an additional toner area coverage. Onthe other hand, when the image data of the pixel arrangement 3302 issubjected to corrective shifting and output as image data of the pixelarrangement 3306 of FIG. 33, the toner-overlapping area of the pixel B(black pixel) and the pixel A (black pixel) does not appear as theadditional toner area coverage any more. As a result, a total toner areacoverage decreases. Hence, the correction-target-pixel selecting unit1502 selects the pixel A or the pixel C as the correction target pixelfrom among the pixels in the pixel arrangement 3306 in the vicinity ofthe shift position of FIG. 33. In this manner, the correction targetpixel is uniquely determined from color density distribution of thepixel of interest and pixels in the vicinity of the pixel of interest.

The density-distribution determining unit 1503 determines colordensities of image data of neighboring pixels that neighbor the pixeldetermined as the correction target pixel by the correction-target-pixelselecting unit 1502 (Step S1514). FIG. 37 is a schematic explanatorydiagram of an example of neighboring pixels for which color densitydetermination is performed. In the first embodiment, thedensity-distribution determining unit 1503 determines a color density (0or 15) of image data of each of four neighboring pixels of thecorrection target pixel. The neighboring pixels are an upper neighboringpixel, a lower neighboring pixel, a left neighboring pixel, and a rightneighboring pixel.

The density correcting unit 1504 corrects the color density of imagedata of the correction target pixel (Step S1515). The density correctingunit 1504 corrects the color density of the image data of the correctiontarget pixel based on an area on the correction target pixel to becovered by a toner image that would be formed based on the color densitydetermined by the density-distribution determining unit 1503. In thefirst embodiment, correction of color density is performed in thismanner. However, an arbitrary technique for compensating an increase ordecrease in color density (toner coverage area) resulting fromcorrective shifting can be employed. FIG. 38 is a schematic explanatorydiagram of the upper, lower, left, and right neighboring pixels of thecorrection target pixel depicted in FIG. 35. FIG. 39 is a schematicexplanatory diagram of pixel sizes of the upper, lower, left, and rightneighboring pixels of the correction target pixel. When, for example,the color density of the lower neighboring pixel is 15 (i.e., blackpixel) and the color density of the other neighboring pixels is 0 (i.e.,white pixel) as depicted in FIG. 38, a toner image of the lowerneighboring pixel is greater than its pixel size. In this case, thelower neighboring pixel has a toner area coverage within the correctiontarget pixel 2001. Meanwhile, density correction of the correctiontarget pixel 2001 does not affect the toner area coverage within thelower neighboring pixel. The density correcting unit 1504 corrects acolor density of image data of the correction target pixel 2001 so as toappropriately compensate an increase or decrease of the toner areacoverage. This correction is performed by correcting the color densityof image data of the correction target pixel 2001 based on an area(toner area coverage) of the correction target pixel 2001 to be coveredby the neighboring pixels on a toner image. The toner image would beformed based on the color density, 15, of the lower neighboring pixel.

FIGS. 40 to 43 are schematic diagrams for explaining how color densitycorrection of image data of a correction target pixel is performed byway of examples.

A color density of image data of a correction target pixel has beenconventionally corrected as depicted in FIG. 40. Specifically, densityhas been corrected such that a toner area coverage of a correctiontarget pixel after density correction attains 95 that is a valuecalculated by subtracting a changed area, 5, from a toner area coverageof the correction target pixel before density correction, 100. Thechanged area is a toner area coverage increased by corrective shiftingand indicated as a hatched area in FIG. 40. When the correction targetpixel is overlapped by a toner area coverage (region α indicated by adotted line) of a neighboring pixel, an actual toner area coverage ofthe correction target pixel after the density correction is an areacovered by the intended post-density-correction toner area coverage andthe changed area. Put another way, the actual toner area coverage of thecorrection target pixel after density correction is not equal to theintended toner area coverage after density correction, 95, that isobtained by subtracting the toner area coverage before densitycorrection, 100, from the changed area, 5. Hence, the conventionaltechnique fails to compensate color density corresponding to theactually changed area of the correction target pixel.

To this end, in the first embodiment, the density correcting unit 1504corrects a color density of image data of a correction target pixelbased on a toner area coverage (region α indicated by a dotted line) ofthe correction target pixel that would be covered by at least oneneighboring pixel. More specifically, the density correcting unit 1504performs density correction such that a portion of the toner areacoverage of the correction target pixel, excluding the region α, beforethe density correction (hereinafter, “pre-density-correction toner areacoverage”) is equal to a portion of the toner area coverage of thecorrection target pixel, excluding the region α, after the densitycorrection (hereinafter, “post-density-correction toner area coverage”)from which the changed area is subtracted.

In the example depicted in FIG. 41, a color density of the image data ofa correction target pixel is corrected so that a toner area coverage ofthe correction target pixel, 85, that is obtained by subtracting thechanged area, 5, from a pre-density-correction toner area coverageexcluding the region α, 90, is equal to 85 that is apost-density-correction toner area coverage excluding the region α, 10.Accordingly, the post-density-correction toner area coverage, 85+10,agrees with a result of subtraction of the changed area, 5, from thepre-density-correction toner area coverage, 100. Hence, a color densitycorresponding to the changed area can be compensated accurately byexcluding a toner area coverage of the lower neighboring pixel on thecorrection target pixel from the toner area coverage of the correctiontarget pixel.

In the example depicted in FIG. 42, the total area of an upper region αand a lower region α is 20. A color density of the image data of acorrection target pixel is corrected so that an area, 75, that isobtained by subtracting the changed area, 5, from apre-density-correction toner area coverage excluding the upper and lowerregions α, 80, is equal to a post-density-correction toner area coverageexcluding the upper and lower regions α, 75. Accordingly, thepost-density-correction toner area coverage, 75+20, agrees with an areaobtained by subtracting the changed area, 5, from thepre-density-correction toner area coverage, 100. Hence, a color densitycorresponding to the changed area can be compensated accurately byexcluding a toner area coverage of the lower and upper neighboringpixels on the correction target pixel from the toner area coverage ofthe correction target pixel.

In the example depicted in FIG. 43, a total area of an upper region α, alower region α, and a left region α is 30. A color density of the imagedata of a correction target pixel is corrected so that an area, 65, thatis obtained by subtracting the changed area, 5, from apre-density-correction toner area coverage excluding the upper, lower,and the left region α, 70, is equal to a post-density-correction tonerarea coverage excluding the upper, lower, and the left regions α, 65.Accordingly, the post-density-correction toner area coverage, 65+30,agrees with an area obtained by subtracting the changed area, 5, fromthe pre-density-correction toner area coverage, 100. Hence, a colordensity corresponding to the changed area can be compensated accuratelyby excluding a toner area coverage of the upper, lower, and leftneighboring pixels on the correction target pixel from the toner areacoverage of the correction target pixel.

How to correct a color density of image data of a correction targetpixel will be described by way of an example.

In the first embodiment, the RAM 123 stores therein color densities ofimage data of upper, lower, left, and right neighboring pixels of acorrection target pixel and density correction values. The colordensities and the density correction values are mapped to each other.The density correction values depend on a toner area coverage of thecorrection target pixel to be covered by a toner image that would beformed based on color densities of the image data of neighboring pixelsof the correction target pixel. The color densities are determined bythe density-distribution determining unit 1503. The density correctingunit 1504 reads from the RAM 123 the density correction value mapped tothe color densities, and corrects the color density of the image data ofthe correction target pixel by using the read density correction value.

FIG. 44 is a table in which color densities of image data of neighboringpixels and density correction values are mapped together. The densitycorrection value depends on an area on the correction target pixel to becovered by a toner image that would be formed based on color densitiesof image data of neighboring pixels of the correction target pixel. Inthe first embodiment, it is assumed that this table is stored in the RAM123 in advance. The density correcting unit 1504 reads a densitycorrection value mapped to color densities of the image data of theupper, lower, left, and right neighboring pixels (hereinafter,“neighboring-pixel color densities”) from the table. The densitycorrecting unit 1504 corrects the color density of the image data of thecorrection target pixel (hereinafter, “target-pixel color density”) byusing the read density correction value. In the first embodiment, it isassumed that a neighboring pixel whose color density is 15 is theneighboring pixel that has a toner area coverage overlapping thecorrection target pixel on a toner image.

It is depicted in FIG. 35 that the target-pixel color density is 15 andcolor densities of its upper, lower, left, and right neighboring pixelsare 0, 15, 0, and 0, respectively. In this case, the density correctingunit 1504 reads the density correction value −4 corresponding to thepattern 4 from the table depicted in FIG. 44, and obtains 11 bysubtracting 4 from the previous color density, 15. The densitycorrecting unit 1504 then corrects the target-pixel color density tothis value, 11. It is depicted in FIG. 36 that the target-pixel colordensity is 15 and color densities of its upper, lower, left, and rightneighboring pixels are 15, 15, 0, and 0, respectively. In this case, thedensity correcting unit 1504 reads the density correction value −5corresponding to the pattern 6 from the table depicted in FIG. 44, andobtains 10 by subtracting 5 from the previous color density, 15. Thedensity correcting unit 1504 then corrects the target-pixel colordensity to this value, 10.

FIG. 45 is a schematic diagram for explaining how a color density ofimage data of a correction target pixel is corrected by way of anexample. Dotted lines indicate a pre-density-correction toner areacoverage. In the first embodiment, because a target-pixel color densityis corrected based on an area on the correction target pixel covered bya toner image that would be formed based on a neighboring pixel of thecorrection target pixel, an increase or decrease of color densityresulting from corrective shifting can be compensated accurately. Forexample, the changed area of a correction target pixel 2602 depicted inFIG. 45 is equal to the changed area of a correction target pixel 2601.However, because correction is performed based on areas on thecorrection target pixels to be covered by toner images that would beformed based on their neighboring pixels, a toner area coverage withinthe correction target pixel 2601 is greater than a toner area coveragewithin the correction target pixel 2602. This is because while thecorrection target pixel 2601 is to be covered only by the toner imagethat would be formed based on its lower neighboring pixel, the tonerimage that would be formed based on the correction target pixel 2602 isto be covered by the toner image that would be formed based on its lowerand upper neighboring pixels.

In this manner, according to the first embodiment, a color density ofimage data of a correction target pixel is corrected based on an area onthe correction target pixel covered by a toner image of at least oneneighboring pixel of the correction target pixel. Hence, an increase ordecrease in the color density resulting from corrective shifting can becompensated accurately.

Meanwhile, the LD-data output unit 133 (133K, 133M, 133C, 133Y) outputsimage data at an output point in time (hereinafter, “output time point”)in a pixel period (cycle) of each pixel during which the image data canbe output. When the density correction is performed on image data of acorrection target pixel by the density correcting unit 1504, the phasecorrecting unit 1505 appends phase data based on which an output timepoint to the image data is to be corrected. Based on the phase data, theoutput time point is advanced or delayed such that the image data isoutput at a position that is displaced from a center positioncorresponding to a center point in time (hereinafter, “center timepoint”) of the pixel period of the correction target pixel toward oraway from the shift position (Step S1516).

In the first embodiment, the phase correcting unit 1505 uses 2-bit phasedata, and appends any one of 00, 10, and 01 to image data. Morespecifically, the phase correcting unit 1505 appends 00 to image datanot to change an output time point of the image data from the centertime point of the pixel period (phase unchanged), appends 10 to delaythe output time point relative to the center time point (leftward phasecorrection), and appends 01 to advance the output time point relative tothe center time point (rightward phase correction).

FIG. 46 is a schematic diagram of an exemplary pixel arrangement in astate before it is subjected to the corrective shifting. FIG. 47 is aschematic diagram of the pixel arrangement of which right side has beenshifted downward. FIG. 48 is a schematic diagram for explaining idealdensity correction for compensating an increase in color density. FIGS.49 and 50 are schematic diagrams for explaining conventional densitycorrection for compensating an increase in color density. When a pixel Bis shifted downward from the state depicted in FIG. 46, the total tonerarea coverage of a pixel (correction target pixel) A and the pixel(correction target pixel) B increase by an area indicated bycross-hatching in FIG. 47. Accordingly, it is ideal to correct a colordensity of the pixel A (pixel B) so as to exclude the area indicated bycross-hatching from the toner area coverage of the pixel A (pixel B) asdepicted in FIG. 48.

However, when the conventional density correction is performed on thepixel arrangement depicted in FIG. 47, the toner area coverage of thepixel A (pixel B) is reduced by bilaterally-symmetrical areas asdepicted in FIG. 49. More specifically, the areas indicated bycross-hatching in FIG. 50 (i.e., portions that do not lead to bandingnoise) are undesirably removed from the toner area coverage. Hence, theconventional density correction fails to increase the toner areacoverage by the area indicated by the dotted lines in FIG. 50 andtherefore fails to bring an image having undergone the densitycorrection close to a not-yet-density-corrected image. This can becomeanother cause of degradation in image quality.

FIG. 51 is a schematic diagram of an exemplary pixel arrangement in astate before it is subjected to corrective shifting. FIG. 52 is aschematic diagram of the pixel arrangement of which right side has beenshifted upward. FIG. 53 is a schematic diagram for explaining idealdensity correction for compensating decrease in color density. FIGS. 54and 55 are schematic diagrams for explaining conventional densitycorrection for compensating a decrease in color density. When a pixel Bis shifted upward from the state depicted in FIG. 51, the total tonerarea coverage of a pixel A (correction target pixel) and the pixel B(correction target pixel) decrease by an area indicated bycross-hatching in FIG. 52. Accordingly, it is ideal to correct a colordensity of a pixel C (pixel D) so as to increase the toner coverage areaby the area indicated by cross-hatching as depicted in FIG. 53.

However, when the conventional density correction is performed on thepixel arrangement depicted in FIG. 52, a toner image of the pixel C(pixel D) having undergone the density correction is formed at a centertime point (phase unchanged) of the pixel C (pixel D). Accordingly, theareas indicated by cross-hatching in FIGS. 54 and 55 (i.e., portionsthat do not lead to banding noise) are undesirably added to the tonerarea coverage. Hence, the conventional density correction fails toincrease the toner area coverage by the area indicated by cross-hatchingin FIG. 53 and therefore fails to bring an image having undergone thedensity correction close to a not-yet-density-corrected image. This canbecome another cause of degradation in image quality.

To this end, the phase correcting unit 1505 performs output-time pointcorrection on image data based on information about an arrangement of atarget correction pixel and its neighboring pixels and a shift directionin shift correction information. The phase correcting unit 1505 receivesthe information about the pixel arrangement from the density correctingunit 1504, and receives the shift correction information from theskew-output control unit 1352. For example, when it is determined that acolor density of image data of a correction target pixel would increasebased on a shift direction and a pixel arrangement of the correctiontarget pixel and its neighboring pixels in shift correction information,the phase correcting unit 1505 advances or delays an output time pointof the image data of the correction target pixel from the center timepoint in the pixel period such that the image data is output at aposition displaced from a center position corresponding to the centertime point toward the shift position.

FIGS. 56 and 57 are schematic exemplary diagrams for explaining outputtime point correction according to the first embodiment performed forcorrecting an increase in color density of image data. As depicted inFIG. 56, the phase correcting unit 1505 delays an output time point ofimage data of the density-corrected pixel A from a center time point ina pixel period of the pixel A so as to output the image data at aposition displaced away from the shift position (leftward phasecorrection). In this case, the phase correcting unit 1505 outputs 10,which is the data for leftward phase correction, as phase data to beappended to the image data of the pixel A. Similarly, the phasecorrecting unit 1505 advances an output time point of image data of thedensity-corrected pixel B from a center time point in a pixel period ofthe pixel B so as to output the image data at a position displaced awayfrom the shift position (rightward phase correction). In this case, thephase correcting unit 1505 outputs 11, which is the data for rightwardphase correction, as phase data to be appended to the image data of thepixel B. This permits the phase correcting unit 1505 to reduce only theareas indicated by dotted lines in FIG. 57 from the toner area coverageof the pixel A and that of the pixel B.

FIGS. 58 and 59 are schematic exemplary diagrams for explainingoutput-time point correction according to the first embodiment performedfor correcting a decrease in color density of image data. As depicted inFIG. 58, the phase correcting unit 1505 advances an output time point ofimage data of the density-corrected pixel C from a center time point ina pixel period of the pixel C so as to output the image data at aposition displaced toward the shift position (rightward phasecorrection). In this case, the phase correcting unit 1505 outputs 11,which is the data for rightward phase correction, as phase data to beappended to the image data of the pixel C. Similarly, the phasecorrecting unit 1505 delays an output time point of image data of thedensity-corrected pixel D from a center time point in a pixel period ofthe pixel D so as to output the image data at a position displacedtoward the shift position (leftward phase correction). In this case, thephase correcting unit 1505 outputs 10, which is the data for leftwardphase correction, as phase data to be appended to the image data of thepixel D. This permits the phase correcting unit 1505 to correct theareas indicated by cross-hatching in FIG. 59 of the toner area coverageof the pixel C and that of the pixel D to the areas indicated bycross-hatching in FIG. 58.

When a bidirectional scanning method with which the exposure unit 9performs rightward horizontal scanning for two colors (for example, Kand M) of Y, M, C, and K and leftward horizontal scanning for the othertwo colors (for example, C and Y) is employed, the write control units126C and 126Y write laterally reversed images. Accordingly, the phasecorrecting unit 1505 outputs laterally-reversed phase data so thatactually-formed toner images of the four colors are appropriatelypositioned.

The corrected-pixel-data output unit 1506 receives the phase data fromthe phase correcting unit 1505 and the image data (density data) fromthe density correcting unit 1504, and outputs the image data, to whichthe phase data is appended, to the write-image processing unit 131 (StepS1517). When the pixel of interest is determined as not being anoise-inducing pixel (No at Step S1512), the corrected-pixel-data outputunit 1506 receives the phase data from the phase correcting unit 1505and the image data (density data) from the data selector 1351, andoutputs the image data, to which the phase data is appended, to thewrite-image processing unit 131 (Step S1517). The LD-data output unit133 outputs LD light-emission signals according to the image data(density data) and the phase data of each pixel. The LD light-emissiondata is used to control light emission from the LDs by using a pulsewidth modulation (PWM) control technique.

In this manner, when density correction is performed on a correctiontarget pixel, the noise-correction processing unit 1353 according to thefirst embodiment corrects an output position at which image data of thecorrection target pixel is to be output by advancing or delaying anoutput time point in a pixel period of the image data. This correctionof the output time point is performed such that the position where theimage data is to be output is displaced from a center position thatcorresponds to a center time point in the pixel period away from ortoward a shift position in a direction toward a portion where localcolor density change does not occur. Accordingly, this correctionpermits, in an image to be formed based on pixels subjected to densitycorrection, a portion that does not lead to banding noise due to skewcorrection to be formed close to a not-yet-density-corrected image.Hence, degradation in image quality resulting from density correctionthat is performed as a part of skew correction can be minimized.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

1. An image forming apparatus comprising: a skew correction unit thatperforms skew correction on image data by dividing pixels that belong toone line in a main-scanning direction of the image data into pixelblocks at least one shift position and shifting a pixel block of thepixel blocks in a sub-scanning direction against a direction of skew; anoise determining unit that determines whether a pixel of interest thatis on the shift position is a noise-inducing pixel, the pixel ofinterest being each pixel on the shift position, the noise-inducingpixel being a pixel that leads to a local color density change becauseof a change in relationship with an adjacent pixel of the noise-inducingpixel; a selecting unit that selects, when the pixel of interest isdetermined as being the noise-inducing pixel, any one of the pixel ofinterest and a pixel in vicinity of the pixel of interest as acorrection target pixel on which color density correction is to beperformed; a first correcting unit that performs the color densitycorrection on density-not-yet-corrected image data of the correctiontarget pixel to acquire density-corrected image data; and a secondcorrecting unit that corrects an output point in time at which thedensity-corrected image data is to be output, the output point in timefalling within a pixel period during which the density-corrected imagedata of the correction target pixel can be output, such that the imagedata of the correction target pixel is output at an output position thatis displaced from a center position corresponding to a center point intime of the pixel period in any one of a direction toward the shiftposition and a direction away from the shift position, wherein: when acolor density of the density-corrected image data is determined todecrease as compared to a color density of the density-not-yet-correctedimage data, the second correcting unit corrects the output point in timesuch that the output position is displaced toward the shift position,and when the color density of the density-corrected image data isdetermined to increase as compared to the color density of thedensity-not-yet-corrected image data, the second correcting unitcorrects the output point in time such that the output position isdisplaced away from the shift position.
 2. The image forming apparatusaccording to claim 1, wherein the second correcting unit determineswhether any one of an increase and a decrease in the color density ofthe density-corrected image data as compared to the color density of thedensity-not-yet-corrected image data would occur based on a pixelarrangement of the correction target pixel and a neighboring pixel ofthe correction target pixel and a shift direction, the shift directionbeing the sub-scanning direction against the direction of skew.
 3. Animage forming method comprising: performing skew correction on imagedata by dividing pixels that belong to one line in a main-scanningdirection of the image data into pixel blocks at least one shiftposition and shifting a pixel block of the pixel blocks in asub-scanning direction against a direction of skew; determining whethera pixel of interest that is on the shift position is a noise-inducingpixel, the pixel of interest being each pixel on the shift position, thenoise-inducing pixel being a pixel that leads to a local color densitychange because of a change in relationship with an adjacent pixel of thenoise-inducing pixel; selecting, when the pixel of interest isdetermined as being the noise-inducing pixel at the determining, any oneof the pixel of interest and a pixel in vicinity of the pixel ofinterest as a correction target pixel on which color density correctionis to be performed; performing the color density correction ondensity-not-yet-corrected image data of the correction target pixel toacquire density-corrected image data; and correcting an output point intime at which the density-corrected image data is to be output, theoutput point in time falling within a pixel period during which thedensity-corrected image data of the correction target pixel can beoutput, such that the image data of the correction target pixel isoutput at an output position that is displaced from a center positioncorresponding to a center point in time of the pixel period in any oneof a direction toward the shift position and a direction away from theshift position, wherein when a color density of the density-correctedimage data is determined to decrease as compared to a color density ofthe density-not-yet-corrected image data, the correcting includescorrecting the output point in time such that the output position isdisplaced toward the shift position, and when the color density of thedensity-corrected image data is determined to increase as compared tothe color density of the density-not-yet-corrected image data, thecorrecting includes correcting the output point in time such that theoutput position is displaced away from the shift position.
 4. The imageforming method according to claim 3, wherein the correcting includesdetermining whether any one of an increase and a decrease in the colordensity of the density-corrected image data as compared to the colordensity of the density-not-yet-corrected image data would occur based ona pixel arrangement of the correction target pixel and a neighboringpixel of the correction target pixel and a shift direction, the shiftdirection being the sub-scanning direction against the direction ofskew.
 5. A non-transitory computer-readable recording medium including acomputer program stored on the readable recording medium, the computerprogram when executed on a computer causes the computer to execute:performing skew correction on image data by dividing pixels that belongto one line in a main-scanning direction of the image data into pixelblocks at least one shift position and shifting a pixel block of thepixel blocks in a sub-scanning direction against a direction of skew;determining whether a pixel of interest that is on the shift position isa noise-inducing pixel, the pixel of interest being each pixel on theshift position, the noise-inducing pixel being a pixel that leads to alocal color density change because of a change in relationship with anadjacent pixel of the noise-inducing pixel; selecting, when the pixel ofinterest is determined as being the noise-inducing pixel at thedetermining, any one of the pixel of interest and a pixel in vicinity ofthe pixel of interest as a correction target pixel on which colordensity correction is to be performed; performing the color densitycorrection on density-not-yet-corrected image data of the correctiontarget pixel to acquire density-corrected image data; and correcting anoutput point in time at which the density-corrected image data is to beoutput, the output point in time falling within a pixel period duringwhich the density-corrected image data of the correction target pixelcan be output, such that the image data of the correction target pixelis output at an output position that is displaced from a center positioncorresponding to a center point in time of the pixel period in any oneof a direction toward the shift position and a direction away from theshift position, wherein: when a color density of the density-correctedimage data is determined to decrease as compared to a color density ofthe density-not-yet-corrected image data, the correcting includescorrecting the output point in time such that the output position isdisplaced toward the shift position, and when the color density of thedensity-corrected image data is determined to increase as compared tothe color density of the density-not-yet-corrected image data, thecorrecting includes correcting the output point in time such that theoutput position is displaced away from the shift position.
 6. Thenon-transitory computer-readable recording medium according to claim 5,wherein the correcting includes determining whether any one of anincrease and a decrease in the color density of the density-correctedimage data as compared to the color density of thedensity-not-yet-corrected image data would occur based on a pixelarrangement of the correction target pixel and a neighboring pixel ofthe correction target pixel and a shift direction, the shift directionbeing the sub-scanning direction against the direction of skew.